The present invention is directed, in general, to an oxidation sensor for an electrical circuit and, more specifically, to a system to provide early warning of oxidation of surface micro-machined Poly-Si MEMS structures.
Micro-electromechanical systems (MEMS) is a well known technology that combines microfabricated electrical circuits with microfabricated mechanical devices such as sensors, valves, gears, mirrors and actuators fabricated on or in semiconductor chips, and is an industry that is growing at an accelerating pace. A MEMS device contains micro-circuitry on a tiny silicon chip into which a mechanical device such as a mirror has been manufactured. The mechanical device moves mechanically under the control of an electrical signal or the device is externally activated and the motion is detected electrically or optically. MEMS systems include systems such as spatial light modulators, deformable mirrors, steerable mirrors, shutters, micro-antennas, relays, and accelerometers. With the rapid commercialization of MEMS, their reliability is of great importance.
In the area of unpassivated MEMS devices, it is recognized that humidity can be an accelerating factor for most phenomenons that may detrimentally affect the devices. Because there is no protective covering on top of the poly-silicon electrodes and wires, they are packaged absolutely hermetically in order to protect the devices from unknown reliability issues.
To prevent the detrimental effects associated with humidity, the industry has designed various packaging systems to isolate the MEMS device from the humidity as much as possible. For example, the industry has developed hermetically sealed ceramic packages that contain the MEMS chip.
In some cases, the hermetically sealed ceramic package can become cracked or damaged sufficiently to allow moisture to enter the package. In such cases, the ingress of moisture within the package can result in operational failure of the device. Moisture may also come from sources inside the package, such as epoxies. The most well known failure phenomenon is stiction, which is the strong interfacial adhesion present between contacting surfaces.
Another failure phenomenon associated with humidity in MEMS devices is oxidation in mechanical systems. If microcracks exists in a system where there is a mechanical device made of silicon, a bit of native oxide can form inside the crack due to the presence of oxygen or water. Any kind of native oxidation, even at the Angstrom or nanometer level will cause a volume expansion inside the crack, which helps to drive the crack further and further into the silicon. The crack can not be closed because the oxide takes up twice the volume of the silicon it consumed. For these reasons, oxidation of silicon can contribute to mechanical reliability issues. This phenomenon has been demonstrated in fracture and fatigue of silicon parts.
Another problem associated with humidity is the reduction of the reflective coefficient of the metals used in optical MEMS devices. The mirrors used in optical MEMS devices are typically made up of a material that reflects light with high reflectivity at the desired wavelength of the light, for example at about the 1000-1600 nm wavelength range for the silicon dioxide optical fiber-based telecommunication systems. Some examples of such reflective materials are gold, silver, rhodium, platinum, copper or aluminum. These reflective metal films are deposited on a movable membrane substrate such as a silicon substrate. Optically, if you have a metal that is subject to tarnishing, then oxidation or moisture can lead to reducing the reflective coefficient of the metal. The reduction of the reflective coefficient of the metal in turn results in the inability to operate the device at its optimum capacity.
Accordingly, what is needed in the art is a system to provide early warning of system failure of polysilicon structures, thereby allowing the operator to replace the leaky package before device failure due to humidity-induced failure phenomena.
To address the above-discussed deficiencies of the prior art, the present invention provides an oxidation sensor for an electrical circuit or MEMS device that includes a conductor located on an insulating substrate and a sensor trace located on the insulating substrate adjacent the conductor. The sensor trace is located on the insulating substrate adjacent the conductor and is configured to oxidize at a rate greater than an electrical component that is associated with the sensor trace when the sensor trace and the electrical component are exposed to a same oxidizing environment. By oxidizing and thus becoming an open circuit more rapidly than any structure on a chip at a given relative humidity, the oxidation sensor is designed to provide early warning of oxidation. Thus, the present invention serves as a sensor that will give advance warning of a leaky or outgassing package and associated oxidation.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.